1. Field of the Invention
This invention relates to projection exposure method and apparatus of the step and scan type in which for example, a mask and a photosensitive substrate are scanned in synchronism with each other relative to an illuminated area of rectangular shape or arcuate shape, whereby a pattern on the mask is exposed onto the photosensitive substrate.
2. Related Background Art
When manufacturing a semiconductive element, a liquid crystal display element, a thin film magnetic head or the like by the use of the photolithography technique, use has heretofore been made of a projection exposure apparatus in which the image of a pattern formed on a photo-mask or a reticle (hereinafter generically referred to as the "reticle") is projected through a projection optical system onto a wafer having photoresist or the like applied thereto or a photosensitive substrate such as a glass plate. The recent tendency is toward making a chip pattern on a semiconductive element large. Accordingly, in the projection exposure apparatus, it is required to expose a pattern of a larger area (make the area larger) on the photosensitive substrate.
In order to meet such a requirement for a larger area, a projection exposure apparatus of the step and scan type has been developed in recent years. In the projection exposure apparatus of this type, each shot area on a photosensitive substrate is stepping-moved to an exposure starting position, whereafter for example, relative to a rectangular, arcuate or hexagonal illuminated area (which is called a "slit-like illuminated area"), a reticle and the photosensitive substrate are scanned in synchronism with each other. Then, patterns on the reticle of larger area than the slit-like illuminated area are successively exposed onto respective shot areas.
Heretofore, in both of a projection exposure apparatus of the above-described step and scan type, and an exposure apparatus of the step and repeat type which is the non-scan type, it has been usual that a focusing mechanism for a reticle pattern disposed on the object plane of a projection optical system and the exposed surface of a photosensitive substrate disposed on the image plane (imaging plane) of the projection optical system is provided only on the photosensitive substrate side. This focusing mechanism is comprised of a detecting sensor for the exposed surface of the photosensitive substrate, and a correcting mechanism for correcting the position of the exposed surface of the photosensitive substrate in the direction of the optical axis of the projection optical system (focus position) and correcting the inclination angle of the surface relative to a plane perpendicular to the optical axis. The detecting sensor detects the focus position of the exposed surface and the inclination angle of the exposed surface. The correcting mechanism corrects the position and inclination of a stage supporting the photosensitive substrate on the basis of the result of the detection by the detecting sensor.
In the projection exposure apparatus of the step and scan type, there are cases where with the scanning of the reticle, the positional relationship between the pattern on the reticle and the projection optical system varies in the direction of the optical axis of the projection optical system. Therefore, the position and inclination of the imaging plane of the projection optical system are also varied. These variations in the imaging plane result in focus deviation.
For example, suppose a 6-inch reticle as the reticle and let it be assumed that the reticle supporting surface of a reticle stage (usually the reticle has its pattern surface side vacuum-chucked and fixed) is upwardly or downwardly inclined by 1 second with respect to the scanning direction. When at this time, the 6-inch reticle is scanned from end to end, the distance between the pattern surface of the reticle and the projection optical system varies by 0.74 .mu.m. To convert this into an amount of variation in the position of the image plane, it can be multiplied by the square of the projection magnification. Supposing a reduction projection optical system having a projection magnification of 1/2+L , the amount of variation in the image plane reaches 0.18 .mu.m. This amount cannot be neglected, particularly when the margin of the depth of focus is small as in the exposure of a pattern of sub-micron order by the projection optical system.